This invention relates to voltage regulation in electric generators and, more particularly, to a power circuit for use in voltage regulators and the method of controlling generator voltage performed by that circuit.
Controlling the output voltage of an electric generator requires control of the generator excitation or exciter field current. Because of the power levels required in some applications, switching regulators are typically used. A typical voltage regulator for aircraft generators is illustrated in U.S. Pat. No. 4,446,417 issued May 1, 1984. In the regulator disclosed in that patent, during normal operation, a positive DC voltage is applied to one end of the generator exciter field winding, and the other end of the winding is connected to ground or, alternatively, to the positive DC voltage by means of a switching transistor. The pulse width modulated duty cycle of the switching transistor controls the average voltage across the exciter field and thus the exciter field current. That current is sensed and converted to a current source signal for feedback to a stability circuit.
If a rapid decrease in exciter field current is desired, such as due to a load decrease, the normal current decay (proportional to the inductance/resistance time constant) is not sufficient. Negative field forcing is used to provide a large negative voltage across the field winding, thereby forcing the current down rapidly. In the circuit of the cited patent, the negative field forcing function is nonlinear. Control of the negative field forcing circuit is provided by an independent control loop which responds to large errors in the output current. Under certain transient conditions, the normal regulation pulse width modulated signal and the negative field forcing circuits can compete for control of the generator output. This situation can degrade transient response.
The present invention provides a generator voltage regulator power stage in which negative field forcing is accomplished without competing with the control circuit used for normal regulation of the generator output.